To help UE access a cell and normally work in the cell, a base station periodically broadcasts system information (SI). The SI is cell-level information. To be specific, the SI is available to all UEs that access the cell. The SI includes system information blocks (SIB). Each SIB includes a parameter configuration set related to one or more functions.
There are two types of cell SI. One type cell SI is a master information block (MIB) with a periodicity of 40 ms. The master information block is repeatedly sent four times within 40 ms, a different redundancy version is used in each time of transmission, and the master information block is transmitted using a physical broadcast channel (PBCH). The PBCH is located on the first four OFDM symbols of the second timeslot in a subframe 0 in the time domain, and occupies six PRBs in the frequency domain. Each PRB occupies 12 subcarriers. REs corresponding to these PRBs can be used to transmit only the MIB, and cannot be used to transmit another message. Message content of the MIB mainly includes cell downlink bandwidth, PHICH configuration, and an SFN (8 most significant bits and 2 least significant bits are determined using four redundancy versions). Another type of cell SI is a SIB. There are a plurality types of SIBs. Currently, a SIB 1, a SIB 2, . . . , and a SIB 19 exist. Each type of SIB is corresponding to parameter configuration of one or some specific functions. All the SIBs are transmitted by using a PDSCH and scrambled by using an SI-RNTI. In other words, the UE receives all the SIBs using one SI-RNTI.
The SIB 1 has a periodicity of 80 ms. In addition, one SIB 1 is repeatedly sent in a subframe 5 of an even number system frame in the periodicity, and this needs to be indicated using a PDCCH in frequency domain. The SIB 1 includes scheduling information of other SIBs. The scheduling information directly indicates SIBs in the cell and periodicities and subframe locations of the SIBs. Each piece of system information may include one or more SIBs. These SIBs have a same transmission periodicity. Each piece of SI is transmitted only in one SI window. First, one piece of SI is associated with one SI window, and the SI window can be used only for sending the SI, and the SI can be repeatedly sent a plurality of times (a quantity of sending times, subframes in which the SI is sent, and the like depend on implementation of the base station), but the SI window cannot be used for sending other SI. Second, SI windows are next to each other, which do not overlap and between which there is no space. Third, all SI has a same SI window length. Fourth, periodicities of different SI are independent of each other.
In comparison with a licensed spectrum cell, an unlicensed cell uses an unlicensed spectrum, and the spectrum is public. The unlicensed spectrum cell may be an LAA cell. The unlicensed spectrum cell can work only when performing carrier aggregation (CA) with the licensed spectrum cell. Alternatively, the unlicensed spectrum cell may be a standalone LAA cell, and can independently work without relying on the licensed spectrum cell. Before sending SI of the unlicensed cell, the base station needs to perform a listen before talk (LBT) process. If LBT succeeds, a SIB and/or a MIB are/is successfully sent; or otherwise, a SIB and/or a MIB cannot be sent. The LBT process means that both the UE and the base station need to perform channel energy detection such as CCA (Clear Channel Assessment) detection before sending data. If energy obtained through detection is less than or equal to a threshold, it is considered that a channel is idle, and data can be sent.
Both the MIB and the SIB are periodically broadcast. If a serving cell has a small quantity of users, when the system information is repeatedly broadcast, a relatively large quantity of radio resources are occupied, and interference is further caused to a serving cell on which channel detection needs to be performed or another unlicensed cell.